Certain aspects of an embodiment of the present invention relate generally to railway car trucks and, more particularly, to a constant contact side bearing to yieldably resist hunting motion of the truck with respect to the car body.
A railroad car typically includes a car body supported on at least one end, and usually two ends, by wheelset trucks that are confined to roll on rails. A standard railroad truck is usually configured in a three-piece arrangement consisting of a pair of laterally spaced sideframes, a bolster extending between the sideframes, and a pair of wheelsets located at opposite ends of the sideframes.
During normal operation, when the railway car is rolling along a straight section of track, the longitudinal axis of each truck bolster is perpendicular to the longitudinal axis of the car body. Each end of the car body is pivotally supported by a truck bolster so that it can rock and swivel relative thereto on a substantially vertical axis. This pivotal connection is typically made by center bearing plates and bowls transversely centered on the car body underframe and the truck bolster, respectively. Accordingly, the truck may turn or pivot on the center plate under the car body and, under certain dynamic conditions and car speeds during operation, the truck may tend to adversely oscillate in a yaw-like manner beneath the car body. This adverse oscillation is commonly referred to in the art as xe2x80x9chunting,xe2x80x9d and it typically occurs when the rail car is lightly loaded and operating at speeds of between 50 and 60 mph.
In order prevent mitigate and reduce hunting, railway freight cars often incorporate devices referred to as constant contact side bearings. The constant contact side bearings are positioned on the truck bolster, outwardly of the center bowl. A constant contact side bearing typically includes a base that is fastened to the top of the bolster and a cap or top that is biased upwardly from the base by a spring so as to contact bearing wear plates (or wedges) on the car body underframe. The constant contact side bearing provides a force between the car body and the truck to frictionally retard the adverse hunting conditions.
Constant contact side bearing are designed to exert a predetermined force at a specified xe2x80x9cset-upxe2x80x9d height. The set-up height measured by the vertical space between the top surface of the bolster and the car body side bearing wear plate (or wedge). At this height, the constant contact side bearing is designed to exert a predetermined force between the truck bolster and the car body underframe. As the cap is compressed towards the base, e.g. due to side to side movement of the car body relative to the truck, the force exerted by the spring increases.
In freight cars the set-up height is typically either 5{fraction (1/16)} (5.0625) inches or 5{fraction (7/16)} (5.4375) inches. As a result, constant contact side bearings are typically designed to have a set-up height which matches one of these common freight car set-up heights. Railway tank cars have a much smaller set-up height than do railway freight cars. A conventional tank freight car has a set-up height on the order of 2xc2xe (2.75) inches or less. As a result, conventional contact side bearings, which have set-up that exceed the set-up heights in conventional railway tank cars, cannot be used in railway tank cars. Hence, there are thousands of railway tank cars in use in the United States and elsewhere that are not equipped with constant contact side bearings. As a result, any train with a tank car must generally travel at reduced speeds to prevent hunting from occurring. Hence, there is a need for a constant contact side bearing that has a reduced height in comparison to prior constant contact side bearings and, in particular, there is a need for a constant contact side bearing that can be used with the reduced set-up height required by conventional tank cars.
According to certain aspects of an embodiment of the present invention, a side bearing provides constant contact between a truck bolster and a railway car body. The side bearing includes a base member having a central portion mounted on top of the bolster. The base member defines pockets extending downwardly and outwardly fore and aft beyond the bolster. A top member is mounted on the base member for vertical movement relative thereto. The top member has a central portion defining a wear pad that is in constant contact with the body wear plate and end portions that overlay the pockets. The central portions of the base and top members are sized so that the side bearing has a set-up height on the order of 2xc2xe (2.75) inches or less, where the set-up height is measured as the distance between the truck bolster and the car body wear plate. According to one embodiment, the base and top members are sized so that the side bearing has a set-up height on the order of 2xe2x85x9c (2.375) inches. Resilient devices are positioned in each of the pockets for urging the top member upwardly relative to the base member. The resilient devices may comprise at least one metallic spring positioned in each of the pockets. According to one embodiment, the resilient means comprises a pair of coaxial springs positioned in each of the pockets. The side bearing components are preferably configured to provide up to ⅝ (0.625) inches of vertical displacement of the top member relative to the base member, although more or less vertical travel can be provided depending on the application.
According to certain other aspects of an embodiment of the present invention, a side bearing for providing a constant contact between a truck bolster and a railway car body includes a base member having a center portion mounted on top of the bolster and end portions extending downwardly and outwardly fore and aft beyond the bolster. A top member overlies and is spaced above the base member. The top member has central portion including a wear pad which engages with the body wear plate. The top member also includes end portions which extend outwardly fore and aft beyond the bolster, and which overlay the base member end portions. The end portions of the top and base members cooperate to form pockets at each end of the base and top members. Resilient devices, such as springs, are positioned in each of the pockets. The resilient devices are configured such that the side bearing provides a predetermined load between the body wear plate and the truck bolster at a predetermined set-up height, where the set-up height is measured between the body wear plate and the truck bolster. The central portion of at least one of the top and bottom members includes a feature for limiting the rotation of the top member relative to the base member. The feature may also limit the downward vertical displacement of the top member on the base member. The feature may comprise a post formed on one of the top member and the base member, where the post is configured to mate with a reciprocal aperture formed on the other member. According to one embodiment, the top member includes a downwardly extending post that mates with a reciprocal aperture formed in the central portion of the base member. The post may be tapered so as to guide the post into the aperture as the top member moves downwardly relative to the base, thereby aligning the top member with the bottom member. The post may further be configured to limit the downward vertical displacement of the top relative to the base sufficiently to prevent the end portions of the top from engaging with the end portions of the base during downward vertical displacement of the top relative to the base. The interface between the post and the aperture also defines a path for force/load transfer between the railway car body and the truck bolster when the side bearing goes solid.